Heretofore, the following properties have been required for a glass substrate to be used for various displays, particularly for a glass substrate for liquid crystal displays designed to have a thin film of a metal or oxide formed on its surface.
(1) It contains substantially no alkali metal ion (i.e. it is alkali-free glass). If a glass substrate contains alkali metal oxides, alkali metal ions are likely to diffuse into the above thin film thereby to deteriorate the properties of the thin film.
(2) It has a high strain point, so as to minimize deformation of the glass substrate and shrinkage (heat shrinkage) accompanied by stabilization of the glass structure when exposed to a high temperature during the process for forming the thin film.
(3) It has adequate chemical durability against various chemicals to be used for formation of semiconductors. Especially, it has the chemical durability against buffered hydrofluoric acid (BHF: hydrofluoric acid+ammonium fluoride) to be used for etching of SiOx or SiNx, chemicals containing hydrochloric acid to be used for etching of ITO (tin-doped indium oxide), various acids (e.g. nitric acid, sulfuric acid) to be used for etching of a metal electrode, or a resist-removing basic liquid.
(4) It has no defects (e.g. bubbles, stria, inclusions, pits or flaws) inside or on the surface.
In recent years, along with the display size being larger, a glass satisfying the following properties as well as the above, has been desired.
(5) A display has been required to have its weight reduced, and glass itself has been also desired to be a glass having a low density.
(6) As a method for reducing the weight of a display, it has been desired to reduce the thickness of a glass substrate.
(7) A glass has been desired to have a linear expansion coefficient which is at the same level as conventional alkali-free glass so that conventional process and facilities can be used in production process of a liquid crystal display.
(8) As liquid crystal TV sets are widely used and are made to have a large size, a glass substrate has been required to have a large rectangular area of at least 2 m on a side from 1 m on a side of a conventional glass substrate. When a display using such a large substrate is to be prepared, handling is difficult due to a large sag by its own weight at the time of transportation and handling, and therefore, the glass substrate has been required to be little sagged.
(9) A glass substrate has been required to have a high strength so as not to be broken by an external force or shock exerted during the use of the liquid crystal display product.
As an alkali-free glass proposed in order to meet such requirements, one disclosed in Patent Documents 1 and 2 may, for example, be mentioned.
Patent Document 1 discloses an alkali-free glass, which has a strain point of at least 640° C. and consists essentially of, as represented by mol %, from 60 to 73% of SiO2, from 5 to 16% of Al2O3, from 5 to 12% of B2O3, from 0 to 6% of MgO, from 0 to 9% of CaO, from 1 to 9% of SrO and from 0 to less than 1% of BaO, provided that MgO+CaO+SrO+BaO is from 7 to 18%.
Further, it is described that such an alkali-free glass can be formed by a float process, white stain due to buffered hydrofluoric acid (BHF) can be prevented, it is excellent in acid resistance, the heat resistance is high, the linear expansion coefficient is low, and the density is remarkably small.
Further, Patent Document 2 discloses an alkali-free glass, which contains essentially no alkali metal oxide and consists essentially of, as represented by mol %, from at least 60 to less than 66% of SiO2, from 0 to 12% of Al2O3, from 5 to 10% of B2O3, from 0 to 18% of MgO, from 0 to 18% of CaO, from 0 to 18% of SrO and from 0 to 6% of BaO, provided that CaO+SrO is from 10 to 25%, and MgO+CaO+SrO+BaO is from 15.5 to 30%.
Further, it is described that such an alkali-free glass is excellent in reduction resistance, whereby it can be preferably formed by a float process, various problems such as deterioration of a devitrification property due to that a glass surface is influenced by a reduction reaction at a time of a float process have been solved, the density is small, the strain point is high, the linear expansion coefficient is low, and the Young's modulus is high.
Patent Document 1: JP-A-9-169539
Patent Document 2: JP-A-2005-330176